This invention relates to a rubber composition which comprises a specific ethylene-.alpha.-olefin-non-conjugated diene copolymer and which is particularly suitable for a sponge.
Ethylene-.alpha.-olefin-non-conjugated diene copolymers are excellent in heat resistance, ozone resistance, weather resistance and the like and sponges obtained by cross-linking and foaming a rubber composition containing said copolymer have been widely utilized as various sealing materials, wire-coating materials and the like. Such sponges have been industrially produced by continuously extruding a rubber composition through an extruder provided with a die having the predetermined shape to obtain an unvulcanized molded article and then vulcanizing and foaming the unvulcanized molded article in a vulcanizer.
However, rubber compositions containing a conventional ethylene-.alpha.-olefin-non-conjugated diene copolymer have such a problem that when vulcanized and foamed by a usual continuous vulcanization method, the rubber compositions tend to be thermally deformed owing to their weights to cause distortion before completion of the vulcanization and foaming and it is difficult to keep the rubber composition in the desired shape. Therefore, the die of the extruder have been made taking the thermal deformation of rubber composition into consideration; however, the making of such a die requires very many steps which have become a severe problem in view of reduction of cost, labor-saving and the like. Moreover, this problem has recently become more important because the crosssectional shape of a sealing material for automobile which is the main use of the sponge has recently become very much complicated and diversified.
In order to improve the shape-retainability of a rubber composition containing an ethylene-.alpha.-olefin-non-conjugated diene copolymer, it has heretofore been tried to combine a high molecular weight component copolymer with a low molecular weight component copolymer. For example, Japanese Patent Application Kokai No. 3-146,531 discloses a rubber composition consisting of copolymer rubber A and copolymer rubber B, the copolymer rubber A having a Mooney viscosity (ML.sub.1+4, 121.degree. C.) of 130 to 195, an ethylene/.alpha.-olefin weight ratio of 73/27 to 40/60 and an iodine value of 8 to 33, the copolymer rubber B having a Mooney viscosity (ML.sub.1+4, 121.degree. C.) of 20 to 55, an ethylene/.alpha.-olefin weight ratio of 73/27 to 40/60 and an iodine value of 10 to 36; and Japanese Patent Application Kokai No. 4-80,245 discloses a rubber composition in which the Mooney viscosity (ML.sub.1+4, 121.degree. C.) is 50 to 100, the ethylene/.alpha.-olefin weight ratio is 73/27 to 40/60, the iodine value is 10 to 36, the low molecular weight component copolymer/high molecular weight component copolymer weight ratio is 51/49 to 80/20 and the ratio of the iodine value of the low molecular weight component copolymer to the iodine value of the high molecular weight component copolymer is 1.1/1 to 4/1.
However, when such two kinds of ethylene-.alpha.-olefin-non-conjugated diene copolymers are blended, the processes for producing the starting copolymers become complicated and it cannot be said that this method is not always advantageous in industry. In addition, conventional rubber compositions containing such two kinds of ethylene-.alpha.-olefin-non-conjugated diene copolymers are still unsatisfactory in an aspect of the balance of characteristics including processibility, shape-retainability, tensile strength, compression set and the like.